tron Paramagnetic Resonance (EPR) spectroscopy is a technique similar to Nuclear Magnetic Resonance (NMR) spectroscopy, with specificity to detect paramagnetic species such as free radicals. Examples of paramagnetic species are molecular oxygen or those generated by exposing tissue to X-rays. Using non-toxic free radical spin probes which are infusible, it is possible to perform EPR imaging (EPRI) similar to the clinically used technique, Magnetic Resonance Imaging (MRI). While MRI non-invasively provides a spatial image of tissue water to represent anatomical image, EPRI provides spatial information (image) of the spin probe distribution in vivo. Since NMR spectra of water protons are relatively invariant in the body and the water content in most tissue is does not vary significantly, anatomical images with high spatial resolution can be obtained. However, since neither the NMR spectral intensity nor the spectral property (line width) vary significantly, it is not straight forward to obtain valuable functional/physiological information using MRI. On the other hand, the EPR spectral properties (line width and intensity) of paramagnetic spin probes are very sensitive to, and are modulated by important physiological properties such as tissue oxygen and redox status. Such EPR spectroscopic information, when spatially encoded in imaging experiments, provides functional/physiological images, which can be co-registered with anatomical images. The feasibility of implementing EPRI in the clinic depends on two factors: 1) biological effects of radiofrequency (RF) radiation, 2) toxicity of the paramagnetic spin probe. Based on the vast information available from the MRI experience on the biological effects of RF radiation, appropriate RF frequency range and power levels can be adapted to carry out EPRI without adverse biological effects. The toxicological properties of the paramagnetic spin probe has been studied at the cellular level as well as in vivo. The tests indicate that the free radical spin probes are well tolerated in vivo and EPRI experiments can be carried out at about 2000 times lower than the maximally tolerated dose. The physiological image information from EPRI will be useful in clinical oncology such as in radiation therapy where treatment of individual tumors can be optimized based on redox status and oxygen status of tumors. Solid human tumors are vascularly compromised and contain zones of hypoxia and become resistant to radiation therapy. Hence, spatial information on the hypoxic zones provided by EPRI could help in radiation treatment planning so that sufficient radiation doses can be delivered to the hypoxic zones to effectively sterilize the tumor. This has been one of the reasons for the Radiation Biology Branch to develop non-invasive functional/physiological imaging methods. The prototype instrument capable of performing small animal studies was available in 1997 and tested with phantom objects. Subsequently, imaging experiments were validated in small objects such as capillaries and the vasculature in the tail of experimental animals. In 1998 the instrument was scaled up for measurements in volumes of 30 - 50 ml, capable of accommodating experimental animals. The RF circuitry, coils, imaging parameters and image reconstruction software were developed and optimized using phantom objects to gain experience and to optimize the technique and the system for in vivo functional/physiological imaging experiments. The experience from phantom EPR imaging experiments suggests the feasibility of obtaining in vivo images from mice and rats with spatial resolution of"